This invention relates to tip treatment bars of a rotor casing for a gas turbine engine.
WO94/20759 discloses an anti-stall tip treatment means in a gas turbine engine, in which an annular cavity is provided adjacent the blade tips of a compressor rotor. The cavity communicates with the gas flow path through the compressor through a series of slots defined between solid tip treatment bars extending across the mouth of the cavity.
Such tip treatments are applicable to both fans and compressors of gas turbine engines, and their purpose is to improve the blade stall characteristics or surge characteristics of the compressor.
Known tip treatments comprise an annular assembly made up of a plurality of segments in which the slots are formed. These segments are provided with tangs which cooperate with slots in the engine casing to hold the segments in position. The assembly may have more than 100 milled slots, and consequently manufacture is expensive and it is difficult to meet the required tolerances.
Experience has shown that tip treatment bars as described above are likely to exhibit cracking due to vibration. This is initiated by the rotor blades as they pass the bars and the subsequent interaction between adjacent bars as they are connected to one another. The proportions of the slots, including their depth and spacing, together with the depth of the annular cavity are aerodynamically important. Analysis of the above arrangement shows a conflict between the mechanical and aerodynamic optimum designs such that either one or the other must be compromised.
An object of the present invention is to avoid cracking of tip treatment bars as a result of vibration. Another object of the present invention is to reduce vibrational coupling between adjacent tip treatment bars.
A further object of the present invention is to provide a tip treatment assembly in which tip treatment bars are installed as separate components.
According to the present invention there is provided a tip treatment bar component for a gas turbine engine, the component having a longitudinal portion and a platform on at least one end of the longitudinal portion, the platform extending laterally to at least one side of the longitudinal portion.
The platform may extend laterally on opposite sides of the longitudinal portion of the tip treatment bar.
In a preferred embodiment, a platform is provided at each end of the longitudinal portion, in which case the platforms may further be connected by a support which is spaced from the longitudinal portion.
A projection, for example in the form of a tang, may extend from the or each platform in a direction away from the longitudinal portion. The projections serve to locate the bar components in an engine casing.
When installed in an engine casing, the bar components form an annular array in which the platforms of adjacent bar components abut each other to maintain spacing between the longitudinal portions. If the bar components include tangs, these extend circumferentially of the array, and may abut adjacent tangs to form a circumferential rib. As the bar components in the array are not rigidly connected to one another, the vibration characteristics of the array are improved. Further, mechanical analysis of the structure is simplified as the bar components are not coupled, and a mechanical solution for the structure is easier to obtain.